This invention relates generally to tuned radio frequency (r.f.) circuits and more particularly to tunable r.f. resonant circuits.
As is known in the art, tunable resonant circuits are often used in receiver applications, such as in a radar receiver system to filter out unwanted frequency components of a signal fed thereto. In particular, bandpass filters having a narrow frequency passband are often used as microwave filters. One approach used in the art to provide a tunable r.f. filter is the use of a resonant circuit including a pair of coupling circuits connected to input and output ports with a ferrimagnetic body disposed adjacent thereto. A YIG sphere is often used as the ferrimagnetic body. The principal of operation with using a YIG sphere as the ferrimagnetic material is that, in the presence of an applied D.C. magnetic field H.sub.DC, the YIG sphere of such material will provide a resonant circuit having a resonant frequency (.omega..sub.o) given as .omega..sub.o =.gamma.H.sub.DC where .gamma. is a quantity referred to as the gyromagnetic ratio. When input energy is fed to an input one of such coupling circuits, a portion of such energy having a frequency substantially equal to the resonant frequency .omega..sub.o is coupled to an output one of such coupling circuits. One problem as known in the prior art is that the resonant frequency of the resonant body in general is a strong function of variations in temperature and thus over the operating temperature range of such resonant circuit such resonant frequency will change. However, in certain orientations of the YIG sphere with respect to the applied d.c. magnetic field H.sub.DC, the resonant frequency of the YIG sphere is independent of variations in temperature over a wide range of operating temperatures. Thus, when fabricating filters using ferrimagnetic bodies, it is most desirable to use an oriented ferrimagnetic body. In some prior art coupling structures, such as a pair of orthogonally spaced semi-circular conductors, it is often required to partially orientate a YIG sphere along an easy axis of the sphere's crystallographic structure, and to complete orientation of the sphere within the presence of such coupling loops to compensate for the spatial uncertainty of the coupling circuits, with respect to the ferrimagnetic body. However, in other applications such as filters employing planar coupling structures where there is no spatial uncertainty between the coupling circuits and the ferrimagnetic body, it is desirable to provide a YIG sphere which is in its final oriented alignment. This is because in order to fully exploit the advantages of high reproducibility and ease of construction generally associated with planar coupling structures, a ferrimagnetic body aligned about its final orientation with respect to a d.c. magnetic field is required in order to minimize additional costly alignment and fabrication steps after assembly of the filter. Thus, in such a case an apparatus and method for aligning a ferrimagnetic body about its final orientation is required.